The tiny bug was that `FnFlags::DEPRECTAED_SAFE_2024` and `FnFlags::RUSTC_ALLOW_INCOHERENT_IMPLS` were assigned the same value.
The catastrophic effect was that every function marked as `#[rustc_allow_incoherent_impl]` was considered safe-deprecated for edition 2024, which caused it to be considered unsafe to call when called from edition 2024. And that includes `<[_]>::into_vec()`, which is called by the `vec![]` macro. So, catastrophic effect.
This innocent-looking bug probably arose from the item tree rewrite. No review would've catch that!
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